How I calculated that the iPhone uses 640MB of mobile data/month

I got a question on how I reached the conclusion, that the iPhone uses on average  640MB of mobile data per month on T-mobile NL.

The T-mobile data I calculated from the combination of two news reports. They seem to have been mentioned by T-mobile in a press release that isn't publicly available. A press release in december does say that mobile data usage has grown 7-times since last year and is now at 2 terabytes per day. 

My calculation was based on data cited on Tweakers.net and Emerce.nl  and goes as follows:

Tweakers says that in December 2008 20,5TB per week was used by mobile users, in June this was 3.1TB. In Januari 2008 this was only 2.5TB per week. If we would have seen a "normal" growth rate, then T-mobile's weekly data usage should have been at 4.5TB maximum. The rest can be attributed to a phenomenon... being the iPhone 3G. 

Emerce.nl reports that there were 100.000 iphone users and also reports they use between 30 to 40 times more data  than normal mobile users. They also report that mobile data users Iphone and other use 67TB of data or 3TB per day. (It looks like they mixed up two numbers here, 67TB/30days is 2TB. So it looks like Tweakers is more accurate. 

So roughly mobile data usage is at 82TB/month, 18 is used by non-iphone users -->64TB is for 100,000 iPhone 3G users= 640MB (and 640 is a magical IT number, so I went with that one)

It's not perfect, but even if we allow for a 20% margin, it's still above half a gigabyte.